1888. J MR. O. THOMAS ON A NEW GENUS OF MURID.E. 1:53 



Madagascar Muridse wholly belong. Thus Cricetine Muridae, now 

 at their highest development in America, were certainly paramount 

 at one time in Africa, where the intermediate Deomijs now occurs, 

 as in other parts of the Old World, and have only recently (since 

 the separation of Madagascar) been supplanted by the more highly 

 specialized Mures. This supplanting, however, has never been quite 

 completed, since in the Old World there still survive several Cri- 

 cetine genera, mostly much modified (e. g. Arvicola and its allies), 

 but in some cases with scarcely any appreciable changes at all 

 (e. g. Mystromys, Cricetus, and, so far as its teeth are concerned, 

 Z,opkiomys). 



And this brings me to the second part of the subject, a part that 

 for reasons of nomenclature is much to be regretted, but which ob- 

 viously has to be investigated, namely, as to the amount of generic 

 distinction actually existing between the Old-World Cricetus and the 

 New-World Hesperomys. So far as I know, they have hardly ever 

 been properly compared, being always presumed to be distinct in the 

 absence of proof to the contrary. On trying to tabulate the dif- 

 ferences, however, and keeping in mind at the same time the extent 

 of variation found in the American species, I find that one by one 

 they vanish into thin air, leaving no distinctive character whatever. 

 The large cheek-pouches of Cricetus are commonly looked upon as 

 a marked characteristic of the genus, but, although small, they occur, 

 distinct and well developed, in many species of Hesperomys, especially 

 in those from the extreme north ^ The teeth oi Cricetus, again, are 

 very like those of many of the species of Hesperomys, especially if 

 those Of" a " Cricetulus " (e. g. C.phceus) be compared with those of 

 some of the species of " Oryzomys " '\ having similarly six paired 

 cusps, while those of " Vesperimus " '' have only five, the cusp 

 A 2 being here obsolete or united with A 1. A still closer re-em- 

 blance, amounting in fact almost to identity, exists between the 

 structure of the teeth in Cricetus and in the Dormouse-like sub- 

 genus Rhipidomys^. In the skull the shape of the infraorbital 

 loramen, of the supraorbital ridges, and of the palate may be equally 

 easily matched among the numerous and widely varying species of 

 '■'■Hesperomys." Finally the peculiar shortness of tail characteristic 

 of Cricetus is almost, it not quite, equalled in the North- American 

 subgenus 0/iychomys'. 



The inevitable conclusion is thus forced upon us that the genus 

 Hesperomys must be abolished altogether, and the species united 



' lu tlie widely spread white-footed Pield-Mouse of N. America (CHce^ws leu- 

 coptis, as it will now have to be called), nortliern specimens have deep and 

 distinct cheek-pouches, while in southern ones they are, at least in spirit-speci- 

 uiens, almost inappreciable. See Allen, Bull. Mus. Comp. Zool. i. p. 229 (18G9), 

 and Coues, Mon. N. Am. Rod. p. 67 (1S77). 



2 E. g. Cricetus pcdusfris or longicaudatus. 



^ Among which are included Cricetus leiicopus, califomious, aureolus, taylori, 

 michlyaiionsis, truei, ajfecus, and mexicanus. 



' tiee, for example, tlie tectli of Cricetus {Nhipidomys) leucodactylus, luiimaHus, 

 suiiuc/zrasti, inustacalis, or sclaleri. 



^ Of which the species are Cricetus leucogaster and C. torridas. 



